Appendix: Quick reference

From %% to end of line.

#, $, @, ~, *, |, \, parentheses, brackets, and braces. Prefix by \ to print.

Object name.

Dictionary word. The @ is optional inside list expressions. Special words to represent non-printable keys: @\n, @\b, @\s, @\u, @\d, @\l, @\r.

Variable.

Number (the valid range is 0–16383).

Empty list.

Complete list.

Partial list.

Closure.

Current topic.

Conjunction.

Negated conjunction.

Disjunction.

Conditions are evaluated at most once. Missing else-clause is assumed to be empty, i.e. succeeds.

Branches entered one at a time, then the final branch repeats.

Branches entered one at a time, then the cycle repeats.

Branches entered at random, avoiding repetition.

Branches entered at random, allowing repetition.

First like (select) …​ (stopping), then like (select) …​ (at random).

First like (select) …​ (stopping), then like (select) …​(purely at random).

Backtracks over all solutions to the statement (typically a block).

Backtracks over all solutions to the inner expression. Values bound to $Element are collected in order and unified with $List.

Backtracks over all solutions to the inner expression, grabbing all output. Printed words are diverted into $List, possibly out of order.

Backtracks over all solutions to the inner expression. Values bound to $Element are added and their sum is unified with $Sum.

Backtracks over every object $Obj that makes the first inner expression succeed, and for which the second expression (when exhausted) emits at least every word in $List.

The inner statement executes, succeeding at most once. The (stop) built-in breaks out of the innermost stoppable environment. The stoppable environment itself always succeeds.

The inner statement executes, succeeding at most once. All output is rendered according to the given style class.

The inner statement executes, succeeding at most once. All output is rendered into a rectangular area according to the given style class.

Like (div $), but the output is rendered into the top status area, which is created if necessary.

Like (div $), but the output is rendered into an inline status area. The previous inline status area, if any, is removed from display.

The inner statement executes, succeeding at most once. The interpreter displays the output, optionally in the form of a hyperlink. If the hyperlink is selected by the player, the output from the inner statement is appended to the current input buffer, which is submitted.

The inner statement executes, succeeding at most once. The interpreter displays the output, optionally in the form of a hyperlink. If the hyperlink is selected by the player, the given $Words are appended to the current input buffer, which is submitted.

The inner statement executes, succeeding at most once. The interpreter displays the output, optionally in the form of a hyperlink. The hyperlink leads to the resource identified by $Id.

If running in the debugger, execute the inner statement in a stoppable environment. The output appears between line breaks, in a distinct style.

Updates a dynamic predicate.

Removes choice-points created since the current predicate was queried.

Declares a global variable.

Declares an interface, i.e. the intended use of a predicate.

Creates $N anonymous objects, for which the predicate will succeed.

Defines an access predicate. Queries or definitions matching the rule head are transformed into the rule body at compile-time.

Unifies $X with $Y.

Succeeds if $X and $Y are numbers, and $X is strictly less than $Y.

Succeeds if $X and $Y are numbers, and $X is strictly greater than $Y.

Unifies $AB with the concatenation of $A (which must be bound) and $B.

Enables bold text.

Succeeds if $X is bound to a value.

Suspends execution (if running in the debugger).

Clears the main window, but not the top status area.

Clears the screen and disables the top status area.

Clears or hides the current div.

Transforms all hyperlinks into plain text, except in the status areas.

Clears the screen from all text that the player has had a chance to read.

Prints the name and version of the compiler.

Displays a backend-specific line of memory usage statistics.

Unifies $Z with the quotient after dividing $X by $Y.

Displays the resource identified by $Id, embedded in the story text.

Succeeds if $X is bound to an empty list.

Fails. Equivalent in functionality to e.g. (1 = 2).

Enables fixed-pitch text.

Succeeds if $X is bound to a value, and—in case of a list—contains only fully bound elements.

Reads a line of input from the player. Returns a list of words.

Waits for the player to press a key. Returns a single-character word.

Dynamic predicate that succeeds when $X is a direct child of $Y in the object tree. Either parameter can be unbound.

Succeeds if the current interpreter supports the resource identified by $Id, and is able to display it using (embed resource $Id) without falling back on just printing the alt-text.

Succeeds if the current interpreter supports inline status areas.

Succeeds if the current interpreter claims to support hyperlinks, and they are currently enabled. This can change at runtime, for instance if a game is saved and subsequently restored on a different interpreter.

Succeeds if the current interpreter supports quit in a way that is meaningful to the player. This can change at runtime, for instance if a game is saved and subsequently restored on a different interpreter.

Succeeds if the current interpreter supports the top status area.

Succeeds if the current interpreter claims to support undo. This can change at runtime, for instance if a game is saved and subsequently restored on a different interpreter.

Unifies $X with each element of $Y in turn.

Enables italic text.

Concatenates the dictionary words (or numbers) in $List into a new dictionary word (or number), and unifies the result with $Word.

Requests a line break.

Succeeds if $X is bound to a list (empty or non-empty).

Unifies $Z with the difference between $X and $Y.

Unifies $Z with the remainder after dividing $X by $Y.

Succeeds if $X is bound to an non-empty list.

Inhibits automatic whitespace before the next word or punctuation mark.

Succeeds if $X is bound to a number.

Checks if $X is an object, or—in a multi-query—backtracks over every object.

Requests a paragraph break.

Draws a progress bar scaled to fit the width of the current div.

Unifies $Z with the sum of $X and $Y.

Immediately terminates the interpreter.

Unifies $Z with a random number in the range $X to $Y inclusive.

Provides an infinite supply of choice points. Generally invoked with a multi-query.

Restarts the program.

Restores a saved game (the interpreter asks for a filename).

Enables reverse-video text.

Disables all text styles (bold, italic, reverse, and fixed pitch).

Saves the current game (the interpreter asks for a filename). Unifies $ComingBack with 0 after a successful save, 1 after a successful restore.

Saves the current program state in memory. Unifies $ComingBack with 0 after a successful save, 1 after a successful restore.

Disables transcription.

Enables transcription (the interpreter asks for a filename).

Prints the serial number (compilation date) of the current program.

Forces whitespace before the next word or punctuation mark.

Prints $N space characters.

Splits $X into two halves around each occurrence of $Y or any member of $Y.

Converts the dictionary word (or number) $Word into a list of its constituent characters, and unifies the result with $List.

Breaks out of the innermost (stoppable) environment.

Unifies $Z with the product of $X and $Y.

Disables query tracing.

Enables query tracing.

Restores the program state at the time of the latest (save undo 0).

Succeeds if $X is bound to a word that wasn’t found in the game dictionary.

Select the default text style for the current division.

Convert the next printed character to uppercase.

Succeeds if $X is bound to a dictionary word.

Execution restarts here when a fatal error has occurred.

Normal execution starts here.

Metadata: Defines the author of the story.

Metadata: Defines the blurb for the story.

Metadata: Defines the IFID of the story.

Metadata: Defines the noun (also known as the headline) of the story.

Metadata: Defines the release number of the story.

Metadata: Defines the title of the story.

Defines the library version and is used to identify the library source code file.

Defines one or more word endings that can be removed when parsing user input.

Associates one or more style attributes with the given class name.

Defines the location (local filename or URL) and alt-text of a resource.